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Why Wheel Aerodynamics Can Outweigh Wheel Weight and Inertia

Written by: Tom Anhalt
Date: Tue May 31 2011

When talking about bicycle wheels, their weight typically receives a lot of attention. This is true for a couple of reasons: First, obviously weight of any sort in bicycle parts receives attention, especially for courses with significant uphill sections, and secondly, most people remember from their high school Physics classes that rotating mass also results in rotational inertia, or a resistance to "spin up". The common wisdom is that wheel mass, since it is rotating, counts "doubly bad" as compared to non-rotating mass. There's a grain of truth to this common wisdom, but it doesn't tell the entire story. In fact, in most cases, wheel mass and rotational inertia tend to be exceedingly small determinants of a particular wheel's performance; much less than the effects of the wheel's aerodynamics, or a particular tire's rolling resistance. If you read most wheel reviews however, there's nearly always a reference to the wheel weight, and in some cases certain reviewers have spent time and money constructing devices to compare wheel inertias and have used that as a main property to rank the wheels against one another. In the example I show below, you'll hopefully begin to see why these sorts of rankings of inertia are actually concentrating on a wheel property that has only a minor affect on wheel performance in the context of the overall bike+rider "system."

Here's the example: I took an actual acceleration of mine from a race file. In this case it was an attack to go for a mid-race prime on a flat section of a crit course. In the span of 5s I accelerated from 25.5 mph to 30.4 mph with an average of 766W expended over those 5s (1s peak of 1080W). Although that's just one of my lowly Cat 4 examples, I think that it's representative of a fairly significant acceleration effort and would be a "worst case scenario" for any wheel inertia effects.

So, I took a look at what the peak forces at the pedal (assumed to be ~2X the average pedal force around the crank cycle) would need to be to create that acceleration due to the different "loads," i.e. merely accelerating the mass, overcoming the increase in aero drag, and also overcoming the increase in rolling resistance with velocity. I then took a look at what a difference in mass of 400g would mean to the situation. It's a somewhat simplistic look at it, but here's how it broke down:
  • Increase in peak force (average over 5s) at the pedal just to accelerate the mass of myself plus the bike = 58.6 lbf. (this is the average ABOVE what it took to go steady state at 25.5 mph).

  • Increase in peak force at the pedal to overcome increased aero drag at end of 5s span = 14.7 lbf.

  • Increase in peak force at the pedal to overcome increased rolling resistance at end of 5s span = 11.5 lbf.

  • Increase in peak force (average over 5s) at the pedal if mass above is increased by 400g = 0.3 lbf.

As you can see, the increase in peak force at the pedal (basically, what you would "feel") is swamped by the total of the other forces…you aren't going to be feeling that mass increase in an acceleration. I even looked at what it would mean if ALL of that 400g increase was at the extreme outer edge of the rims and it's effects on the rotational inertia and the additional force needed to "spin up" that additional mass. Doing that changes that last bullet from 0.3 lbf to 0.9 lbf…3X worse, but still minuscule in the grand scheme of things.

As can be seen in the pie chart to the left, the vast majority of the increase in peak force felt at the pedal was due to just linearly accelerating my (and my bike's) mass. Additionally, the increases in aero drag and rolling resistance accounted for approximately 1/3 of of the total increase in peak pedal force. The effect of increasing the rim mass of the wheels by nearly a full pound (400g) resulted in an increase in the average peak pedal force over the 5s acceleration of just 1%…in other words, an amount not likely to be "felt" at the pedals. It's pretty clear which areas of wheel performance the most benefit can be gained from making improvements: in the aerodynamics of the wheels and the rolling resistance of the tires and tubes applied to them.

  

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Comments

Acceleration and Aero contribution 4 out of 5 stars

Reviewed by: Dan Phillips, Jul 12 2011 10:05AM

You claim that acceleration at any given speed is going to require the same forces at the crank, but whilst accelerating a mass through 5mph will require a set amount of energy for any initial velocity, this is surely under the assumption that fluid resistance is negated. Given that P = f(v^3) (http://en.wikipedia.org/wiki/Drag_(physics)#Power) then an increase of speed brings a cubic increase in the power required to overcome drag.

What about handling? 2 out of 5 stars

Reviewed by: Eric, Jul 11 2011 4:11AM

The article fails to mention a wheel's impact on handling. Whether turning the wheel into a corner or rocking the bike side to side on a climb, the rotational inertia will have an effect. How much is a question the author should have addressed.

Paul, I think you're missing the point... 5 out of 5 stars

Reviewed by: Tom Anhalt, Jun 22 2011 1:39PM

The article doesn't claim that weight doesn't matter for pure hillclimbs...it does and gravity is the major source of resistance in that case since speeds are low enough that aerodynamic drag is a small part of the total. However, the article was addressing being able to "feel" the mass and rotational inertia during an acceleration on basically level ground or on a rolling type course (where aerodynamic drag is MUCH higher), and it's in that scenario that "aero trumps mass".

There is a difference and it can be significant - depending on the type of road and climb 4 out of 5 stars

Reviewed by: paul casino, Jun 22 2011 7:23AM

Tom and Jordan, since you guys believe riders can't really "feel" the difference and that we should all provide empirical data to bolster our opinions between various wheel masses; I will drive my point that you could "feel" it by giving an extreme situation. Imagine racing up the famed Mt. Washington during its annual climb to the top race. Would you not "feel" the difference between using Zipp 202s (front and rear) versus using a Zipp 1080 rear with an 808 front? The aerodynamic difference between these two sets of wheels are quite big, and so is the weight difference. But please tell me that you cannot "feel" the difference between the two options. I know no one will use a 1080/808 wheelset to climb Mt. Washington - PRECISELY for obvious reasons that the total mass of these two wheels completely negates its extreme aerodynamic character. For schmozes like myself (unlike pros like yourself) we do feel the weight difference of wheelsets in climbing harder hills.

Wow...looks like there's some good responses here... 5 out of 5 stars

Reviewed by: Tom Anhalt, Jun 11 2011 7:29PM

...and I'd like to address some of the comments/questions/suggestions:
- a roller test might be a good way of figuring out the "cost" of just the wheel inertia differences, except it leaves off the major source of inertia...the rider itself. The physics is pretty simple to calculate the forces required based on just the masses and geometry, so I'm not seeing how roller testing is going to tell us much. The high acceleration case I used in the analysis, along with the worst case assumption of ALL of the mass being added to the outer circumference of the wheels only increased the peak force at the pedal by less than 1 lbf.
- After reading the article, does anyone really think they'll be able to "feel" a difference in acceleration from moving 30g of nipple mass from the rims to the hubs? Sure, it'll change the rotational inertia of the wheels...but the fact remains that the rotational inertia of the wheels is an exceedingly small portion of the total inertia and forces on the bike+rider system.
- Although the speed was 25-30mph, that's not what's important...it's the acceleration that's important because that's what results in a force difference at the pedal. Whether it's from zero mph, or above 20 mph, the same rate of acceleration will cause the same peak force differences at the pedal.
- Lastly, I didn't say that mass doesn't matter...I'm only saying that when selecting wheels, wheel mass and inertia shouldn't be one of the first things you worry about.

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